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Hexagonal boron nitride (h-BN) is a layered material that is generating interest in various fields owing to its fascinating properties. We present a multi-stranded analysis of carbon defects in h-BN using intentionally carbon (C)-doped h-BN crystals. Carbon defects were introduced into h-BN crystals synthesized under high-pressure and high-temperature (HPHT) conditions by a process of carbon annealing. We employ an innovative approach to explore impurity states in our C-doped h-BN by assembling it into van der Waals heterostructures with other 2D materials. Attaching graphene as a probe material to determine the impurity states in the h-BN bandgap, we locate an acceptor state above the Dirac point of graphene on C-doped h-BN, which causes anomalous bending in the Landau fan diagram. Furthermore, we adopt tungsten disulfide (WS2) as another probe material and assess the influence of C-doped h-BN on the emission spectrum of the WS2 monolayers. Our work reveals the nature of the carbon impurities in h-BN and also uncovers their effects on adjacent 2D materials.

Hexagonal boron nitride (h-BN) crystals grown under ultrahigh pressures and ultrahigh temperatures exhibit a high crystallinity and are used throughout the world as ideal substrates and insulating layers in van der Waals heterostructures. However, in their central region, these crystals have domains which contain a significant density of carbon impurities. In this study, we utilized cathodoluminescence and far-ultraviolet photoluminescence to reveal that the carbon (C)-rich domain can exist even after exfoliation. Then, we studied the carrier transport of graphene in h-BN/graphene/h-BN van der Waals heterostructures, precisely arranging the graphene to straddle the border of the C-rich domain in h-BN. We found that the carrier mobility of graphene on the C-rich h-BN domain was significantly suppressed. In addition, characteristic bending of the Landau fan diagram was observed on the electron-doped side. These results suggest that the C-rich domain in h-BN forms an impurity level and induces extrinsic carrier scattering into adjacent graphene.

Hexagonal boron nitride (h-BN) synthesized under high pressure and high temperature (HPHT) has been used worldwide in two-dimensional (2D) materials research as an essential material for constructing van der Waals heterostructures. Here, we study h-BN synthesized with another method, i.e., via synthesis at atmospheric pressure and high temperature (APHT) using a metal alloy solvent. First, we examine the APHT h-BN in a bulk crystal form using cathodoluminescence and find that it does not have carbon-rich domains that inevitably exist in a core region of all the HPHT h-BN crystals. Next, we statistically compare the size of the crystal flakes exfoliated on a SiO2/Si substrate from APHT and HPHT h-BN crystals by employing our automated 2D material searching system. Finally, we provide direct evidence that APHT h-BN can serve as a high-quality substrate for 2D materials by demonstrating high carrier mobility, ballistic transport, and Hofstadter butterfly in graphene and photoluminescence in WS2.